Gas-insulated transmission line

ABSTRACT

A gas-insulated transmission line compatible with operation from low through EHV range. Insulating supports are provided for maintaining the position of the inner conductor within the outer conductor while permitting longitudinal movement of the inner conductor in response to thermal buildup. Expansion joints connecting sections of the inner conductor adapt to longitudinal changes of the central conductor consistent with desirable electrical and mechanical operation. Couplings connecting adjacent outer conductors are designed for simple installation yet with sufficient rigidity to assure gastight operation.

United States Patent [72] inventor [21] Appl.

22 Filed [45] Patented [54] GAS-INSULATED TRANSMISSION LINE John George Trump 3,324,272 6/1967 Shankle et al. 174/28 X 9 Cambridge 81., Winchester, Mass. 01890 3,356,785 12/ 1967 Yasuhisa Yoshida et al. 174/28 N01 749,135 3,361,870 H1968 Whitehead 174/88 X July 31,1968 3,391,243 6/1968 Whitehead 174/28 June FOREIGN PATENTS 274,597 11/1928 Italy 174/28 Primary Examiner- Lewis H. Myers ABSTRACT: A gas-insulated transmission line compatible with operation from low through EI-IV range. Insulating supports are provided for maintaining the position of the inner conductor within the outer conductor while permitting longitudinal movement of the inner conductor in response to thermal buildup. Expansion joints connecting sections of the inner conductor adapt to longitudinal changes of the central conductor consistent with desirable electrical and mechanical operation. Couplings connecting adjacent outer conductors are designed for simple installation yet with sufficient rigidity 17 Claims, 16 Drawing Figs. Assistant Examiner-A. T. Grimley 52 us. Cl 174/13, Ammeyhhw'n Aishaw Franc [51] Int. Cl H0lb 9/06 50 Field of Search ,174/15 0,

13, 12, 16B, 28, 29, 69,99 E, 88 B, 25 G [56] References Cited UNlTED STATES PATENTS 2,044,580 6/1936 174/28 2,191,071 2/1940 174/28 2,428,051 9/1947 174/28 2,432,568 12/1947 174/25 X 3,236,933 2/ 1966 Frowein 174/13 to assure gastight operation.

70 0,6; & m\

PATENTEU JUN] 519m sum 2 or 5 FIG. 3

10; FIG. 4

PATENTEUJUNISIHH 5 5,270

sum 3 0F 5 I Q I G 7 V Q' g2 I 4 III 7. FIG.8

PATENTED JUN] 519m SHEET 5 BF 5 GAS-INSULATED TRANSMISSION LINE BACKGROUND OF THE INVENTION It is well known that a significant trend exists toward the transmission and utilization of ever larger amounts of electric power. At the present time, the transmission of power to large urban population centers has conventionally been by means of overhead lines from the point of generation to the point of distribution and usage. The few exceptions to this reside in the furnishing of power to downtown areas and in some of the newer and more exclusive suburban areas. in the former instance, underground power transmission has been a necessity as a result of crowded conditions, limited easements and rightof-ways, and safety factors where higher voltage and power loads have been required. in the latter instance, however, the driving force behind underground cables has been more due to aesthetic continuity of the living area as opposed to technical and practical limitations as the public has been reluctant to accept the influence of high voltage overhead transmission lines. In addition, there are both technical and economic limitations to the underground transmission of small amounts of power by conventional high voltage oil-paper and solid dielectric cables.

The solid-insulated cable is typical of the underground cable now in .use. This cable comprises an inner conductor having a solid insulation built up from spiral-wrapped paper and subjected to a vacuum-drying process followed by thorough impregnation with insulating oil or similar voidfilling material. These solid insulated cables are used for voltages as high as 345 kv. but are inefficient since they carry only a fraction of the current capability of an overhead line operating at this voltage and at far higher cost per unit power transmitted.

For more than a score of years, consideration has been given to compressed gas-insulated transmission lines as an alternative to overhead lines for both preserving the beauty of the countryside, as well as providing increased power transmission at comparable costs. Moreover, the technical aspects of the gas-insulated transmission line offers unique advantages for present and future needs of underground electric power transmission. For example, in contrast to oil-paper cable systems, a rigid gas-insulated transmission line offers:

1. Substantially reduced charging current and increased permissible length of line without relative compensation due to the low dielectric constant of gas which is essentially unity even at high pressures. Moreover, the electrode geometry of rigid concentric conductors can be made more favorable than in the care of flexible oil-paper cable. This can result in overall reduction in the comparative capacitance by a factor of about four.

2. Low dielectric loss and lower conductor resistance especially when the compressed gas is operated below the ionization level. Thus, the dielectric loss under these conditions, even at high gradients, is of negligible amount. The use of a rigid high voltage conductor permits and encourages the use of larger cross sections with corresponding gains in current and heat transfer capabilities and with greater choice of conductor materials. As a result, power capabilities surpassing those of present overhead lines of the same voltage are possible improved thermal performance arising from the superior heat transfer properties of compressed gas. This is a result of the nondegrading dielectric properties of gas with temperature, the permissible use of temperature insensitive spacer materials and the lower thermal resistance to earth of the larger external pipe.

4. Voltage insulating properties which can be applied to all present voltage levels and which can clearly be extended to 1000 kv. 10,000 megawatt capacities.

However, previous attempts to economically provide such lines have been frustrated by structural problems related to manufacture of the line, difficulties in maintaining the proper radial position of the inner conductor within the outer conductor or gas container, and in joining of adjacent transmission line sections to provide proper rigidity and exclude contamination. In addition, previous gas-insulated transmission lines have not been able to compensate properly for thermal expensation of the central conductor when under heavy load and problems have arisen in attempts to assure gastight seals between connecting transmission line sections that will resist leakage even when the insulating gas is pressurized. Then, too, electrical problems have arisen relative to providing a sufficient electrical connection between the central conductors, maintaining a predetermined gradient in the insulating space between the conductors, and minimizing sparkover where supports are required in the line. It is the failure of the prior art to cope with these and other factors that has delayed the commercial realization and embodiment of gas-insulated transmission line.

In an effort to understand the insulating properties of gases at supra-atmospheric pressures and the mechanisms of insulation failure, much research has been performed during the last several decades. However, this work has been largely done on relatively small electrodes immersed in compressed gases and has done much to clarify the major factors which influence the insulating strength of this medium. For example, in small electrode systems, a semiquantitative understanding of the influence on DC breakdown voltage of the nature and pressure of the gas, the material of the electrodes, the area and smoothness of the electrode surface, the interelectrode geometry and the polarity has been developed. This experimental information has taught that a given system may occasionally spark or are at voltage which is several times lower than that which can be insulated at another time. But, since sparking on a transmission line would be catastrophic, this spread or variability in performance is a serious difficulty that remained unresolved by the prior art and must be overcome in any workable embodiment.

indeed, until the development of the present invention the understanding and performance of compressed-gas insulation has not been under sufficient control to permit a design with the high predictable reliability required for either AC or DC operation of an actual high voltage, high power transmission line installation.

SUMMARY OF THE INVENTION it is, therefore, a general object of the present invention to provide a new and improved gas-insulated transmission line compatible with transmitting power up to the most demanding power'requirements in the EHV range.

Another object of the present invention is to provide anew and improved gas transmission line which minimizes dielectric loss.

A further object of the present invention is to provide a new and improved gas-insulated transmission line which allows continuous acceptable operation during longitudinal expansion and contraction of the inner conductor in response to varying power loads.

An addition object of the present invention is to provide a new and improved gas-insulated transmission line which may be manufactured in component or modular units for subsequent assembly in the field.

A still further object of the present invention is to provide a new and improved gas-insulated transmission line wherein the inner conductor is supported within the outer conductor by strategically placed supports which maintain a predetermined electrical gradient compatible with desired operating conditions.

Yet a further object of the present invention is to provide a new and improved gas-insulated transmission line wherein couplings and connectors of adjacent sections of outer and inner conductors are consistent with a predetermined structural and electrical reliability.

In accordance with the general principles of the present invention, a gas-insulated transmission line is provided having an inner conductor supported periodically by insulator supports within an outer conductor which also serves as a gas container when the insulating gas is pressurized. The line is assembled by joining premanufactured sections in the field and introducing a gas-insulating medium to the region between the inner and outer conductors. Although one very desirable gas is sulfur hexafluoride, it may be desirable for economic reasons to use other gases such as nitrogen, carbon dioxide, or mixtures of these gases. Pressurizing the insulating gas within the line to greater than one atmosphere provides greater insulating strength as a function of the type of gas and the pressure. Because of thermal expansion caused by current losses, the inner conductor is periodically joined by expansion joints which permit expansion and contraction in response to the thermal changes in the inner conductor which give rise to its longitudinal motion. These expansion joints provide an electrical and mechanical connection between connecting inner conductors and have electrical characteristics at least equaling that of the conductors themselves. The insulator-supports for the inner conductor provide either a fixed position for both the outer and inner conductor, or permit the movement of one conductor relative to another. The geometry and dielectric properties of the supports produce a predetermined electric field distribution which results in a voltage insulating strength preferably equal to or greater than that of the gas itself. Finally, the outer conductors are joined by gastight couplings which allow for admission, pressurization and removal of gas even after installation.

Other objects and advantages of the present invention will become apparent in view of the following description taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE FIGURES FIG. I is a schematic longitudinal section of interconnected sections or modules of a single phase gas-insulated transmission line.

FIGS. 2, 3 and 4 are detailed longitudinal sections of various embodiments of expansion joints for connecting inner conductors, and couplings for connecting outer conductors.

FIG. 5 shows a detailed longitudinal section of an expansion joint for connecting adjacent inner conductors where the expansion joint is not coincident with an outer conductor coupling.

FIG. 6 shows in radial cross section another embodiment of an expansion joint whereby the connecting apparatus is entirely within the inner conductor and motion is achieved by the flexing of resilient webs.

FIG. 7 is a longitudinal section along line 9-9 of FIG. 6.

FIG. 8 is a longitudinal section along line -10 of FIG. 6.

FIG. 9 illustrates how a higher density of resilient webs per unit length may be achieved.

FIG. 10 is a longitudinal section of another embodiment of an inner conductor expansion joint using resilient webs.

FIG. 11 is a longitudinal section of a fixed post-type support which permits longitudinal movement of the inner conductor.

FIG. 12 illustrates in radial cross section a fixed post-type support.

FIG. 13 shows in longitudinal cross secton a post-type support which permits longitudinal movement of the inner conductor.

FIG. 14 shows aradial section of a post-type support fixed to the inner conductor and movable on the inner surface of the outer conductor.

FIG. 15 illustrates a longitudinal section of a movable annular support fixed to the inner conductor and movable on the inner surface of the outer conductor.

FIG. 16 is a diagrammatic representation of a three-phase gas-insulated transmission line which is electrically cross-connected to reduce the effects of induced voltages.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, a typical view in longitudinal section is shown of a gas-insulated transmission line having an inner conductor IC positioned within an outer conductor or gas container 0G,. The space or region between the two conductors is filled with an insulating gas 34 such as sulfur hexafluoride which may be at greater than atmospheric pressure. Ideally the inner conductor IC, would remain suspended within the outer conductor 0C as illustratedby the basic line portion A which would assure a uniform voltage gradient throughout the length of the line. Of course, as a practical matter, the inner conductor IC, must be supported within the outer conductor 0C by various types of supports designated generally as D, E and F. On these supports D, E and F rests the integrity of the line-both mechanically and electrically.

In addition to radial support of the inner conductor IC,, the problems associated with longitudinal expansion of the inner conductor IC must be considered. More particularly, when large amounts of current are carried by a conductor, the finite resistance in the conductor causes a thermal buildup with resultant expansion. The longitudinal component of the expansion is proportional to the length of conductor used. Thus the supports D, E and F must be designed to provide radial support for the inner conductor IC as well as provision for longitudinal expansion. This is achieved by having certain of the supports F fixed to inhibit relative movement between the inner conductor IC,, and other supports D and E which are themselves movable or allow movement of the inner conductor IC relative to the outer conductor OCI Yet, whatever the type of support, they should contribute only a minimum degree of perturbation of the electric field and should have surface and volume breakdown strength equal to or exceeding that of the surrounding gas. Moreover, the expansion joint E in addition to assuring proper radial support throughout longitudinal changes without excessive mechanical stress provides an electrical connection 30 between the adjacent rigid ends of the sections of the inner conductor IC, The external geometry of the electrical connection 30 should not adversely affect the voltage-insulating strength of the system in that region. Furthermore, the cross-sectional area of the electrical connection 30 should be at least equal to that of the inner conductor IC provided the same conductor material is used. On the other hand, if another conductor material of equal crosssectional area is used, it should have a sufficiently low resistivity to prevent a hot spot in the inner conductor IC Compliance with these requirements assures proper operation and long life of the expansion joint E which is an economic necessity. These considerations necessarily preclude conventional solutions such as sliding contacts which would weld together under short circuit conditions or create a hot spot in the line.

A detailed description of each of the various types of supports will be discussed later herein.

The construction of the transmission line of FIG. 1 may be accomplished both at a manufacturing facility and in the field. Generally, standard length section or modular units P P -P of the line would be prepared and assembled in the factory by inserting the required number of supports D, E and F and one or more sections of the inner conductor lc into the outer conductor 0G,.

A standard length section is desirable to take advantage of the standard tube lengths presently manufactured and to provide a completed subassembly which is adaptable to common means of transportation.

The preassembled standard length sections with their ends sealed against vapor or particle contamination together with sufficient couplings would subsequently be deposited along the route of transmission. The sections would then be coupled together in the field to make a field joint B by connecting inner conductor to inner conductor and outer conductor to outer conductor thereby forming an integral, rigid unit.

In joining preassembled standard length sections of the line such as P,, P P clean conditions are necessary to inhibit introduction of contaminating particles such as dirt, dust, fibers and metal filings. Experience has shown that such particles would adversely effect the insulation strength of the finished line. Thus, it is highly desirable, both from an economic and operational standpoint, to provide the simplest and most reliable type of field joint having a minimum number of joining structures.

By way of example, at the field joint B, an alignment plug 36, of a material having a conductivity and cross section at least equal to that of the inner conductor, is introduced into the hollow ends of adjacent sections 3l and 33 of the inner conductor IC Once the alignment plug 36 is in place, the two abutting ends 31, 33 of the inner conductor IC, may then be welded at 38 to provide a rigid structure and smoothed to minimize electrical weakness. Other methods of joining the inner conductors may also be employed with the objective of attaining electrical continuity and sufficient conductivity throughout the length of the insulated and supported inner conductor as well as adequate surface contact at all rigid and elastic joints to carry full load current continuously without overheating and fault currents (20 times operating current for 2 seconds) without thermal damage. For example, sawtoothed members of sufficient surface area and conductivity may be mated together and attached (not shown).

Once the inner conductors have been joined, a coupling sheath C used to couple the two sections 35, 37 of the outer conductor C,, is slipped over the conductor OC and welded thereto at 40. Other coupling devices will be hereinafter described all having a simple attachment and providing rigidity between outer conductor sections Both the coupling sheath C and the alignment plug 36 could be designed to introduce changes in direction whether azimuthal or vertical, or a combination thereof.

Inasmuch as the inner conductor IC, is not a perfect conductor, the increasing current acting on the finite resistance of the inner conductor IC causes a thermal buildup which is be into longitudinal expansion. The degree of expansion is both a function of the thermal buildup and the coefficient of expanding of the conductor material. For example, a 40 foot length of aluminum will expand about 1 inch over a 100 C temperature rise. To accommodate the longitudinal changes in the inner conductor IC a system of fixed, movable or partially movable insulating supports and expansion joints are required. One such system, for example, is illustrated in FIG. 1, where three preassembled standard length sections, P P P are joined as part of a total transmission line. Fixed supports are assembled on like ends of sections P, and P and provide for a rigid connection between inner conductor IC and outer conductor 00,, An expansion joint E is positioned generally near the 5 of section P and accommodates both ends 39, 41 of the inner conductor IC whether or not in an expanded state. Thus in each section, as in assembled section P longitudinal expansion of the inner conductor IC, will be directed inwardly toward the expansion support EL Interposed between the fixed supports F and the expansion support at line portion E are movable or partially movable supports D which maintain the radial position of the inner conductor IC, within the outer conductor OC, as longitudinal movement of the inner conductor IC occurs. In embodiments where movement results in a sliding action of the support against a conductor, it is preferable to have the support slide against the outer conductor to restrict such movement to the lower gradient region.

A detailed embodiment of an expansion support is illustrated in FIG. 2. The inner conductor IC may be supported within an outer conductor coupling C or just within the outer conductor itself (not shown) by an dish-shaped insulator-support I made of glass, porcelain, epoxy or other solid dielectric material. The insulator l may be corrugated on the outer surfaces to increase the electrical length or creepage distance. Reentrant grooves or channels 42a and 44a are shown respectively on the outer and inner peripheries of the insulator-support I: and are coated with a conductive surface 46a. Application of this conductive surface 46a may be by a firing-on process or even by painting on an adherent conductive coating. Suffice it to say, the conductive coating 46a must cover the entire surface of the reentrant channels and must either be beaded at the channel edges or extended slightly beyond the channel to assure a positive electrical contact when physical contact with the associated conductor is intended. In this manner no potential difference will exist in the reentrant channels between the conductive surface and the corresponding conductor. Moreover, the high gradient will necessarily be drawn away from all the insulator extremities in contact with the inner conductor such as 47, a type of juncture which is otherwise recognized as fundamentally weak at high voltages. Equipotential lines demonstrate a fairly uniform radial gradient along the lateral surface of insulator If at the expense of somewhat higher gradients in the volume of the insulator I.,.

The insulator I accommodates two end sections 43, 45 of the inner conductor lC A plurality of thin conducting loops or strips 48 of a conductor are then used to electrically connect these end sections 43, 45. Conductive spacers 50 are sandwiched between the ends of the conducting loops 48 to provide a rigid connection to the end sections 43, 45. At no load conditions the thin loops will be substantially on the same axis at the inner conductor IC But, with an increasing load and subsequent longitudinal expansion the thin conductive loops will bow within the inner reentrant channel 44. As such there is no perturbation to the electric field inasmuch as the bowed thin loops are constrained within a low field region.

In accordance with the invention disclosed and claimed in my copending application Ser. No. 601,945, filed Dec. 15, I966, now abandoned, a dielectric coating 52a of glass, ceramic, or epoxy is applied over the inner rendered conducting on its inner surface 540 adjacent to the inner conductor lC Although in principle, the conductive coating, which may be of any appropriate material may be as thin as one molecular layer of anodized aluminum, it may preferably be increased to the order of one-eighth or one-fourth inch. However, the thickness of the dielectric coating 52 must still be small in comparison with the remaining gas-filled gap, e.g. the distance between the inner conductor IC: and the outer conductor 0C2.

The shell or wall of the outer conductor 0C must be of nonmagnetic material to avoid excessive series inductance on AC transmission lines. The inner surface of the outer conductor 0C may also benefit by being coated with a relatively thin dielectric coating (not shown). However, the voltage gradient is less at the inner surface of the outer conductor 0C than at the outer surface of the inner conductor IC so that the dielectric coating may be omitted at the former location.

Other than simply welding the outer sheath coupling C to the outer conductor Oc as demonstrated in FIG. 1, detailed arrangements for another coupling apparatus is shown in FIG. 2 and designated by C Multiple O-rings or gaskets 55a of either organic or inorganic material, may be provided to inhibit pressure leakage when the insulating medium is pressurized. In addition, test gaskets 58 are provided and through a connection 56 interposed between test gaskets 58 and gaskets 55, leakage of the gas-insulating medium can be easily detected.

FIG. 3 has all the previously described features of FIG. 2; however, the inner diameter of the coupling C; has been extended somewhat to accommodate the insulator I which has a diameter somewhat greater than the inside diameter of the outer conductor 0C This enlargement effectively permits a lower gradient intensification near the coupling C which reduces the electric stresses at the point of contact of the insulator I and the coupling C The coupling C has a valve 60 which provides for the introduction or removal of the insulating medium without requiring dismantling of the coupling C;,.

In FIG. 4, another embodiment of an expansion joint is shown. The coupling C, has been split along its longitudinal axis with an extension portion 62 at each longitudinal edge. Bolts 64 are then used to hold the two halves of the coupling C, together. A gasket 65 prevents leakage of the insulating medium at the coupling C The insulator I, has a greater thickness near the outer conductor C than at the inner conductor IC and cooperates with sealing gaskets 61 and 63 to act as a pressure barrier between adjacent sections of the transmission line.

It should be noted that the embodiments of expansion supports shown in FIG. 2, 3 and 4 and previously described herein are not restricted by their structure to placement at a coupling of outer conductor sections. Indeed, these embodiments could advantageously be used at any internal point in a transmission line section and, in particular, at the midpoint of a section as shown schematically by E in FIG. 1.

Another slightly modified embodiment of an expansion joint having aninterior placement within a transmission line is shown in FIG. 5 incorporating most of the features previously shown and described in FIGS. 2, 3 and 4. Bolts 66 are used to clamp the flexible conductor loops 48b and the conductive spacers 50b to the end sections 43b, 45b. Longitudinal slits 68, 69 at respective end sections 43b, 45b allow compression when the bolts 66 are tightened. A metal insert 70 is introduced into the outer reentrant channel 42b such that when the insulator-support I is in place, the outer conductor 0C, may be strategically crimped at 72 to inhibit movement of the insulator support 1 In addition to its electrical function of connecting conductive layer 46b to the outer shell OC,, the metal insert 70 is used as a safety measure to distribute the pressure over a larger area and prevent a possible cracking of the insulator I, if it were otherwise used alone. Connections 18 are made between the flexible conductor 48b and the conductive surface 46b on the reentrant channel 44b of insulator support I, to provide an easy low loss path for inductive currents.

Thus far in this description expansion joints have been associated with a coincident insulator support. This configuration has been necessary to control and confine the high potential gradient around the inner conductor and is unquestionably a necessity at EHV ranges. Yet another type of expansion joint wherein no surrounding insulator is required is shown in section in FIGS. 6, 7 and 8. The outer conductor 0C completely surrounds and is substantially concentric with portions L and R of the inner conductor IC A pair of conductive adapters 88, 90 are mounted within the inner conductor IC,, and are preferably of a conducting material. The adapters then hold respective side members 92, 94 also of a conducting material. Electrical and mechanical connections between the side members 92, 94 is provided by a plurality of resilient webs 96 which will be discussed in detail later herein. It should be noted that the connection between inner conductor sections L and R is at all times confined within the hollow center portion of the inner conductor IC, and thus needs no reentrant support structure to insure a proper gradient between inner and outer conductors.

Both the left-hand section L and the right-hand section R have a greater than half cylindrical section removed to allow the extending portion of one to fit into and mate with the removed portion of the other thereby being a substantially closed cylinder. The maximum longitudinal gap 93 between the conductor section L and R should be only slightly greater than the total possible expansion which may occur as a result of thermal buildup. All edges at the gap 93 should be rounded to eliminate electrical stress concentration.

A detailed showing of the expansion joint structure is illustrated in FIG. 8. A plurality of resilient webs 96 are connected by soldering, cold-forging, etc. in an arched fashion between the side members 92, 94. Relative longitudinal movement of the inner conductor sections is then accomplished by the flexing of these resilient webs 96. Consideration must be given to the mechanical stress and strain placed on each web during movement. Indeed, stress must be below the yield point by sufficient margin to escape fatigue and below the stress which causes material hardening or other change in properties of the material.

If the thickness of each web is t and the radius is r, then the outer radius of each web is 1+1 and the strain on each web is then given by the following:

Strain Arc outside-are inside 2 Stress is proportional to strain and may be given by:

Stress 2 r where E modulus of elasticity Thus, knowing the diameter of the inner conductor and the thickness of the web, a conductive material can be chosen which has the necessary required modulus of elasticity. To eliminate the possibility of a hot spot at an expansion joint, the net conductivity of the webbed assembly should be equal to or greater than that of the inner conductor.

Longitudinal motion occurs by movement of either or both of the side members 92, 94. For example should side member 94 move in the direction of the arrow 100, point P on the web which was previously on that portion of the web adjacent and parallel to the side member 94 upon movement becomes a part of the are at P. Similarly, point Q on the opposite side of the web, which was previously on the arc of the web, now moves to Q, a point wherein the web is adjacent and parallel to the side member 92. To increase the range of movement, the distance between side members 92 and 94 can be decreased or a greater portion of the web may be made parallel to the side members 92, 94.

In order to improve rigidity in a compressed gas insulated transmission line it is desirable to minimize the length of the expansion joints. However, limitations upon size are controlled by providing a necessary and proper number of webs to maintain a cross-sectional conductivity equal to that of the inner conductor. To increase the density of webs 96 per unit length of side member and thus reduce the length of the expansion joint, the webs have been attached as shown in FIG. 9. By chamfering the bottom inner corner 102 of the web 97, the subsequent web 99 can then be attached a distance d closer to the previous web than would otherwise be without the chamfer.

In FIG. 10, another of the web-type expansion joints is shown similar to that of FIG. 8. In this embodiment, however, the conducting webs 107 are mounted normal to the side members 106, 108. Although the sliding movement is less than would otherwise be possible with the previously described webs, a normal mounting would provide a simple assembly and easy web attachment The adapters 110, 112 are held in compression between the inner conductor 30 and respective side members 106, 108 by conducting bolts 114. For economic reasons the adapters 110, 112 may be tapered or stepwise decreased (not shown) from a point of maximum thickness near the contact with the first web to a point of minimum thickness near the contact with the first web to a point of minimum thickness near the contact of the last web since, as more webs are encountered, the greater will be the current flow through them. In FIG. 6, the same effect could be achieved by progressively eliminating the extending portions 107 on the adapters 88, 90.

FIG. 11 shows a detailed embodiment of a fixed type support designated by F in FIG. 1. More particularly FIG. 11 shows a support having an annular insulator I with inner and outer reentrant channels 440, 42c having a conductive coating 460. A low field region exists between the conductive coating 46a and the respective abutting conductors. Included within these reentrant channels are a plurality metal inserts 116, 118 having a synclinally depressed outer surface which will accommodate crimping of the inner and outer conductors at 120 and 122 respectively to maintain them fixed relative to one another. All of the previously described electrical characteristics related to annular supports are applicable herewith.

Now referring to FIG. 12, a post-type insulator I is shown supporting a flattened inner conductor 1C A receptacle 123 is provided in the top of the insulator I to which a conductive coating 46d has been applied. Within this receptible 123 a metal insert 126 receptacle provided which accommodates a bolt 127 securing the inner conductor. A reinforcing place 131 is used between the head of bolt 127 and the inner conductor IC Similarly, at the bottom of the insulator I another receptacle 128 is provided to which the conductive coating 46d is applied. Making a rigid connection with the a receptacle 128 is a metal insert 129 to which a bolt 130 is attached for securing the outer conductor C to the insulator I thereby providing a rigid and fixed mounting of the inner conductor IC Provision of the metal inserts 126, 129 and the conductive coating 46d serve to reduce the electrical gradient in the critical gaseous regions close to the insulator-conduc tor-junctions.

A significant advantage of the post-type insulator is in its ease of assembly relative to the annular type. The inner conductor with attached post insulators is easily insertable into the outer conductor without scratching the inner surface and may be easily mounted into predrilled holes 134 in the outer conductor OC and secured by the bolt 130. It should be noted that the outer conductor 0C has been extended somewhat at 135 to permit connection of the insulator assembly in a low. field region. Although this may not be neces sary to a proper connection, it advantageously removes a potentially troublesome area to an area of lowest gradient. A weld 149 is then made around the bolt-outer conductor contact to prevent pressure leakage of insulating medium. The inner conductor IC may be advantageously slotted longitudinally at 132 and mounted slightly below center to pump dust particles into the longitudinal slot 132. Reference is hereby made to my copending Pat. application Ser. No. 653,152 entitled Dust Precipitators," allowed Feb. 19, 1970, setting forth in detail the dust collection technique. Electrically, the posttype insulator has all the advantages of the annular type with the additional advantage that the contact surface between insulator and inner conductor is considerably reduced and its insulating length is increased.

Extending the concept of the post insulator, in FIG. 13 a support for permitting longitudinal motion of the inner conductor is shown corresponding to slide support D of FIG. 1. The post insulator I has an upper receptacle 146 and a lower receptacle 148 which have an applied conductive coating 46c. In the upper receptacle 146 a metal insert 152 is bonded to the insulator I and has an extending stud 154 which may be inserted through a predrilled hole 156 in the outer conductor OC A reinforcing plate 158 is placed over the stud 154 and a nut 160 secures the entire structure to the outer conductor 0C During tightening of the nut 160, the reinforcing plate 158 acts to distribute the force over a greater surface area on the outer conductor 0C as well as to inhibit twisting of the entire insulator assembly. Gastight welds 162 insure for the possibility of using a pressurized insulating medium.

The lower receptacle receives a metal insert 164 which has bonded therein a laminar conducting member 166 which extends through a longitudinal slot 170 in the inner conductor 1C and is fastened to the inner surface of the inner conductor 1C by bolts 168. The laminar conducting piece 166 is flexible and permits either rightward or leftward longitudinal movement of the inner conductor 1C without radial displacement. As previously mentioned, the combination of a longitudinal slot 170 along with an inner conductor having a radial position slightly below center will make possible the continuous electrostatic removal of conducting and semiconducting particles by precipitating them into the hollow center conductor through the longitudinal slot therein under the continued action of any applied voltage. This embodiment also provides for maximum longitudinal movement (by mounting the laminar conductor piece 166 to the lower inner surface of the inner conductor 1C while eliminating any sliding frictional movement. Since the laminar piece 166 is conducting, the metal insert 164 and the conductive coating 46:: on the lower receptacle 148 are at the same potential thus improving the electric field distribution by reducing the field intensity at the insulator-conductor region. It should be noted that a laminar conducting member could also be used in the upper receptacle (not shown) to provide even greater longitudinal freedom of movement.

FIG. 14 is another embodiment of a support permitting longitudinal movement of the inner conductor relative to the outer conductor. An insulator I is attached to the inner conductor IC and both are movable within the outer conductor OC A metal insert 172 is provided within the outer receptacle 174 and serves as mechanical support for rollers which move along the inner surface of the outer conductor 0C during longitudinal movement of the inner conductor 1G,... The horizontal portion of inner conductor 1C is held by a bolt 182-to the inner receptacle 176 of the insulator I Both the receptacles 174 and 176 have a conductive coating 46f. This arrangement also assists in reducing gradients near the inner conductor and traps dust in accordance with my previously mentioned patent application Ser. No. 653,152.

In FIG. 15, a movable insulator I is shown which is rigidly connected to the inner conductor 1C for movement along the inner surface of the outer conductor 0C The insulator I is annular in shape and has respective inner and outer reentrant channels 44g and 42g with an applied conductive coating 46g. Within the inner reentrant channel 44g is an annular metal insert 184 having a continuous synclinal depression 186 which receives a crimped portion 188 of the inner conductor [C to form rigid connection therebetween. A plurality of metal inserts 190 is included in the outer reentrant channel 423 each having depressed portions 192 for receiving a spring 194 or other suitable bias member for holding rollers 196 against the outer conductor 0C,,,. Longitudinal movement of the inner conductor would result in a rotation of the rollers 196 permitting the insulator assembly to move relative to the outer conductor OC,,,.

Because of induced currents, undesirable voltages along the gas container pipes will develop. To minimize these voltages in three-phase transmission, the gas containers may be crossconnected at various points along their lengths and generally whenever the induced voltage approaches 65 volts. In FIG. 18, an electrical cross-connection scheme for a predetermined length of three-phase transmission line is shown when the phases are designated by PI-l Ph and P11 Although the gas containers are connected together and to ground at each end, intermediate connections are provided for all permutations between phases. The couplings C insulate one section of pipe from another and with the insertion of an elastomeric band could be compatible with couplings C C and C in FIGS. 2, 3 and 4.

Although the present invention has been described with a certain degree of particularity, it should be understood that the present disclosure has been made only by way of example and that numerous changes in the details of construction and the combination of said outer arrangement of parts may be resorted to without departing from the scope and spirit of the present invention. For example, various combinations of flexible, sliding and fixed supports may be used for maintaining integrity of each section of the transmission line and similarly allowing predetermined operational loads.

Iclaim:

1. In a gas-insulated transmission line comprising at least one modular unit, the combination of:

a. a nonmagnetic outer conductor,

b. a nonmagnetic inner conductor,

c. means for insulatedly supporting said inner conductor within said outer conductor, and wherein said support means is responsive to longitudinal expansion of said inner conductor to maintain the longitudinal axis of said inner conductor substantially parallel to the longitudinal axis of said outer conductor,

wherein said supporting means comprises:

a. at least one fixed support for inhibiting longitudinal motion of said outer conductor relative to said inner conductor,

b. at least one movable support rigidly mounted to one of said conductors and permitting longitudinal motion of the other conductor,

where said support means are solid insulators, disclike in shape, and having at least one reentrant channel on at least one peripheral surface of said insulators, said reentrant channel having applied thereon an electrically conducting layer which is electrically coupled to the adjacent conductor, and wherein said transmission line also includes expansion means positioned within the reentrant channel on the inner periphery of a support and electrically coupling adjacent end sections of said inner conductor, said expansion means being responsive to changes of length'of inner conductor between fixed supports.

2. The gas-insulated transmission line as set forth in claim 1 wherein said expansion means comprises a plurality of flexible conductors having a conductivity approximating that of the said inner conductor, the opposite ends of said flexible conductors being connected to adjacent inner conductor ends.

3. In a gas-insulated transmission line comprising at least one modular unit, the combination of:

a. a nonmagnetic outer conductor,

b. a nonmagnetic inner conductor,

c. means for insulatedly supporting said inner conductor within said outer conductor, and wherein said support means is responsive to longitudinal expansion of said inner conductor. to maintain the longitudinal axis of said inner conductor substantially parallel to the longitudinal axis of said outer conductor,

wherein said supporting means comprises:

a. at least one fixed support for inhibiting longitudinal motion of said outer conductor relative to said inner conductor,

b. at least one movable support rigidly mounted to one of said conductors and permitting longitudinal motion of the other conductor,

wherein said supports include solid insulators, disclike in shape, and having at least one reentrant channel on at least one peripheral surface of said insulators, wherein each reentrant channel of said insulator has applied thereon an electrically conducting layer which is electrically coupled to the adjacent conductor, wherein at least one conducting means is inserted into at least one of said reentrant channels thereby making mechanical and the electrical contact between one of said conductors and said applied conducting layer, and wherein said conducting means has a synclinal-shaped depression for receiving a crimped portion of a conductor to thereby clamp the support associated therewith to said conductor.

4. In a gas-insulated transmission line comprising at least one modular unit, the combination of:

a. a nonmagnetic outer conductor,

b. a nonmagnetic inner conductor,

c. means for insulatedly supporting said inner conductor within saidouter conductor, and wherein said support means is responsive to longitudinal expansion of said inner conductor to maintain the longitudinal axis of said inner conductor substantially parallel to the longitudinal axis of said outer conductor,

where the outer conductor of adjacent modules of said transmission line are connected by gastight couplings, and wherein said couplings are designed to introduce changes in direction, whether azimuthal or vertical or combinations thereof.

5. In a gas-insulated transmission line comprising at least one modular unit, the combination of:

a. a nonmagnetic outer conductor,

b. a nonmagnetic inner conductor,

c. means for insulatedly supporting said inner conductor within said outer conductor, and wherein said support means is responsive to longitudinal expansion of said inner conductor to maintain the longitudinal axis of said inner conductor substantially parallel to the longitudinal axis of said outer conductor,

wherein said supporting means comprises:

a. at least one fixed support for inhibiting longitudinal motion of said outer conductor relative to said inner conductor,

b. at least one movable support rigidly mounted to one of said conductors and permitting longitudinal motion of the other conductor,

and wherein said transmission line further includes expansion means comprising a plurality of flexible conducting straps being of sufficient number and of conductivity to maintain the mechanical and electrical integrity of the transmission line throughout its range of operation.

6. The gas-insulated transmission line as set forth in claim 5 wherein a greater than half cylindrical section is removed from each end section of said inner conductors for a predetermined length and wherein the remaining cylindrical portion of one end section fits into the removed portion of the other end section and thereby aligning the remaining cylindrical portion of one section opposite the remaining cylindrical section of the other.

7. The gas-insulated transmission line as set forth in claim 5 wherein the end section of said adjacent inner conductors each comprise:

a conductive adapter mounted on the inner surface of each of said inner conductor ends for adapting a respective inner conductor surface .to said expansion means,

a conducting side member mounted on each of said conductive adapters; and wherein said flexible conducting straps are attached at opposite ends to each of said conducting side members.

8. The gas-insulated transmission line as set forth in claim 7 wherein each of said conductive adapters proceeds from a minimum cross-sectional area nearest the end of said inner conductor to a maximum cross-sectional area at its opposite end.

9. The gas-insulated transmission line as set forth in claim 5 wherein said flexible conducting straps each form an arcuate and wherein opposite ends of said flexible conducting straps are tangent at their juncture with a respective end section.

10. The gas-insulated transmission line as set forth in claim 5 wherein said flexible conducting straps form a center arcuate portion between two straight portions which are normal at their juncture with a respective end section.

11. In a gas-insulated transmission line comprising at least one modular unit, the combination of:

a. a nonmagnetic outer conductor,

b. a nonmagnetic outer conductor,

c. means for insulatedly supporting said inner conductor within said outer conductor, nd wherein said support means is responsive to longitudinal expansion of said inner conductor to maintain the longitudinal axis of said inner conductor substantially parallel to the longitudinal axis of said outer conductor,

wherein said supporting means comprises:

a. at least one fixed support for inhibiting longitudinal motion of said outer conductor relative to said inner conductor,

b. at least one movable support rigidly mounted to one of said conductors and permitting longitudinal motion of the other conductor, and

wherein said movable support comprises:

a post-type insulator having a receptacle at each end,

a conducting layer applied to the surface of said receptacles,

a first conducting means inserted within one of said receptacles,

means for fastening said first conducting means to one of said conductors to provide electrical and a rigid mechanical contact,

a second conducting means inserted within said other receptacle and having a flexible conducting member con-' necting said other conductor and said second conducting means such that a predetermined longitudinal motion of the inner conductor is permitted.

12. The gas-insulated transmission line as set forth in claim 11 wherein said conductor has at least one aperture for attachment to an interior point on the inner conductor wall opposite said aperture.

13. The gas-insulated transmission line as set forth in claim 11 wherein said fastening means comprises a flexible conducting member connected on the one end within said first conducting means and on the other end to the inner surface of said outer conductor.

14. In a gas-insulated transmission line comprising at least one modular unit, the combination of:

a. a nonmagnetic outer conductor,

b. a nonmagnetic inner conductor,

c. means for insulatedly supporting said inner conductor within said outer conductor, and wherein said support means is responsive to longitudinal expansion of said inner conductor to maintain the longitudinal axis of said inner conductor substantially parallel to the longitudinal axis of said outer conductor,

wherein said supporting means comprises:

a. at least one fixes support for inhibiting longitudinal motion of said outer conductor relative to said inner conductor,

b. at least one movable support rigidly mounted to one of said conductors and permitting longitudinal motion of the other conductor,

wherein said fixed support comprises:

a post-type insulator having a receptacle at each end,

a conducting layer applied on the surface of said receptacles,

at least one conducting means within said receptacles; and

fastening means for maintaining the conducting means within each receptacle in electrical and mechanical contact with one of said conductors, and

wherein said fastening means comprises:

a stud member extending away from said insulator means and adapted to fit through a hole in said outer conductor a reinforcing plate member having a curvature substantially equal to that of the outer diameter of said outer conductor and having a hole for admitting said stud member,

securing means cooperating with said stud member for providing a rigid and gastight connection of said stud member and reinforcing plate to said outer conductor.

15. The gas-insulated transmission line as set forth in claim 11 wherein said first conductor fastening means comprises:

a stud member extending away from said insulator means and adapted to fit through a hole in said outer conductor,

a reinforcing plate member having a curvature substantially equal to that of the outer diameter of said outer conductor and having a hole for admitting said stud member; and

securing means cooperating with said stud member for providing a rigid and gastight connection of said stud member and reinforcing plate to said outer conductor.

16. In a gas-insulated transmission line comprising at least one modular unit, the combination of:

a. a nonmagnetic outer conductor,

b. a nonmagnetic inner conductor,

c. means for insulatedly supporting said inner conductor within said outer conductor, and wherein said support means is responsive to longitudinal expansion of said inner conductor to maintain the longitudinal axis of said inner conductor substantially parallel to the longitudinal axis of said outer conductor,

wherein said supporting means comprises:

at least one fixed support for inhibiting longitudinal motion of aid outer conductor relative to said inner conductor,

b. at least one movable support rigidlymounted to one of said coductors and permitting longitudinal motion of the other conductor,

wherein said fixed support comprises:

a post-type insulator having a receptacle at each end,

a conducting layer applied on the surface of said receptacles,

at least one conducting means within said receptacles; and

fastening means for maintaining the conducting means within each receptacle in electrical and mechanical contact with one of said conductors, and wherein a predetermined portron of outer conductor has a larger diameter than the remaining portion of said outer conductor to create a low intensity field region and wherein the fastening means between said outer conductor and the associated conducting means is in said larger diameter portion.

17. The gas-insulated transmission line as set forth in claim 11 wherein said movable support comprises:

a post insulator having a receptacle at each end, a conducting layer applied to the surface of said receptacles, first conducting means inserted within one of said conductors to provide an electrical and a rigid mechanical contact, second conducting means inserted within the other of said receptacles and having translational contact means cooperating with said other conductor to permit relative motion between conductors while said second conducting means is maintained in electrical contact with said other conductor. 

1. In a gas-insulated transmission line comprising at least one modular unit, the combination of: a. a nonmagnetic outer conductor, b. a nonmagnetic inner conductor, c. means for insulatedly supporting said inner conductor within said outer conductor, and wherein said support means is responsive to longitudinal expansion of said inner conductor to maintain the longitudinal axis of said inner conductor substantially parallel to the longitudinal axis of said outer conductor, wherein said supporting means comprises: a. at least one fixed support for inhibiting longitUdinal motion of said outer conductor relative to said inner conductor, b. at least one movable support rigidly mounted to one of said conductors and permitting longitudinal motion of the other conductor, where said support means are solid insulators, disclike in shape, and having at least one reentrant channel on at least one peripheral surface of said insulators, said reentrant channel having applied thereon an electrically conducting layer which is electrically coupled to the adjacent conductor, and wherein said transmission line also includes expansion means positioned within the reentrant channel on the inner periphery of a support and electrically coupling adjacent end sections of said inner conductor, said expansion means being responsive to changes of length of inner conductor between fixed supports.
 2. The gas-insulated transmission line as set forth in claim 1 wherein said expansion means comprises a plurality of flexible conductors having a conductivity approximating that of the said inner conductor, the opposite ends of said flexible conductors being connected to adjacent inner conductor ends.
 3. In a gas-insulated transmission line comprising at least one modular unit, the combination of: a. a nonmagnetic outer conductor, b. a nonmagnetic inner conductor, c. means for insulatedly supporting said inner conductor within said outer conductor, and wherein said support means is responsive to longitudinal expansion of said inner conductor to maintain the longitudinal axis of said inner conductor substantially parallel to the longitudinal axis of said outer conductor, wherein said supporting means comprises: a. at least one fixed support for inhibiting longitudinal motion of said outer conductor relative to said inner conductor, b. at least one movable support rigidly mounted to one of said conductors and permitting longitudinal motion of the other conductor, wherein said supports include solid insulators, disclike in shape, and having at least one reentrant channel on at least one peripheral surface of said insulators, wherein each reentrant channel of said insulator has applied thereon an electrically conducting layer which is electrically coupled to the adjacent conductor, wherein at least one conducting means is inserted into at least one of said reentrant channels thereby making mechanical and the electrical contact between one of said conductors and said applied conducting layer, and wherein said conducting means has a synclinal-shaped depression for receiving a crimped portion of a conductor to thereby clamp the support associated therewith to said conductor.
 4. In a gas-insulated transmission line comprising at least one modular unit, the combination of: a. a nonmagnetic outer conductor, b. a nonmagnetic inner conductor, c. means for insulatedly supporting said inner conductor within said outer conductor, and wherein said support means is responsive to longitudinal expansion of said inner conductor to maintain the longitudinal axis of said inner conductor substantially parallel to the longitudinal axis of said outer conductor, where the outer conductor of adjacent modules of said transmission line are connected by gastight couplings, and wherein said couplings are designed to introduce changes in direction, whether azimuthal or vertical or combinations thereof.
 5. In a gas-insulated transmission line comprising at least one modular unit, the combination of: a. a nonmagnetic outer conductor, b. a nonmagnetic inner conductor, c. means for insulatedly supporting said inner conductor within said outer conductor, and wherein said support means is responsive to longitudinal expansion of said inner conductor to maintain the longitudinal axis of said inner conductor substantially parallel to the longitudinal axis of said outer conductor, wherein said supporting means comprises: a. at least one fixed support for inhibiting longitudinal motion of said outer conductor relative tO said inner conductor, b. at least one movable support rigidly mounted to one of said conductors and permitting longitudinal motion of the other conductor, and wherein said transmission line further includes expansion means comprising a plurality of flexible conducting straps being of sufficient number and of conductivity to maintain the mechanical and electrical integrity of the transmission line throughout its range of operation.
 6. The gas-insulated transmission line as set forth in claim 5 wherein a greater than half cylindrical section is removed from each end section of said inner conductors for a predetermined length and wherein the remaining cylindrical portion of one end section fits into the removed portion of the other end section and thereby aligning the remaining cylindrical portion of one section opposite the remaining cylindrical section of the other.
 7. The gas-insulated transmission line as set forth in claim 5 wherein the end section of said adjacent inner conductors each comprise: a conductive adapter mounted on the inner surface of each of said inner conductor ends for adapting a respective inner conductor surface to said expansion means, a conducting side member mounted on each of said conductive adapters; and wherein said flexible conducting straps are attached at opposite ends to each of said conducting side members.
 8. The gas-insulated transmission line as set forth in claim 7 wherein each of said conductive adapters proceeds from a minimum cross-sectional area nearest the end of said inner conductor to a maximum cross-sectional area at its opposite end.
 9. The gas-insulated transmission line as set forth in claim 5 wherein said flexible conducting straps each form an arcuate and wherein opposite ends of said flexible conducting straps are tangent at their juncture with a respective end section.
 10. The gas-insulated transmission line as set forth in claim 5 wherein said flexible conducting straps form a center arcuate portion between two straight portions which are normal at their juncture with a respective end section.
 11. In a gas-insulated transmission line comprising at least one modular unit, the combination of: a. a nonmagnetic outer conductor, b. a nonmagnetic outer conductor, c. means for insulatedly supporting said inner conductor within said outer conductor, nd wherein said support means is responsive to longitudinal expansion of said inner conductor to maintain the longitudinal axis of said inner conductor substantially parallel to the longitudinal axis of said outer conductor, wherein said supporting means comprises: a. at least one fixed support for inhibiting longitudinal motion of said outer conductor relative to said inner conductor, b. at least one movable support rigidly mounted to one of said conductors and permitting longitudinal motion of the other conductor, and wherein said movable support comprises: a post-type insulator having a receptacle at each end, a conducting layer applied to the surface of said receptacles, a first conducting means inserted within one of said receptacles, means for fastening said first conducting means to one of said conductors to provide electrical and a rigid mechanical contact, a second conducting means inserted within said other receptacle and having a flexible conducting member connecting said other conductor and said second conducting means such that a predetermined longitudinal motion of the inner conductor is permitted.
 12. The gas-insulated transmission line as set forth in claim 11 wherein said conductor has at least one aperture for attachment to an interior point on the inner conductor wall opposite said aperture.
 13. The gas-insulated transmission line as set forth in claim 11 wherein said fastening means comprises a flexible conducting member connected on the one end within said first conducting means and on the other end to the inner surface of said outer conductor.
 14. In a gas-insulated transmission line comprising at least one modular unit, the combination of: a. a nonmagnetic outer conductor, b. a nonmagnetic inner conductor, c. means for insulatedly supporting said inner conductor within said outer conductor, and wherein said support means is responsive to longitudinal expansion of said inner conductor to maintain the longitudinal axis of said inner conductor substantially parallel to the longitudinal axis of said outer conductor, wherein said supporting means comprises: a. at least one fixes support for inhibiting longitudinal motion of said outer conductor relative to said inner conductor, b. at least one movable support rigidly mounted to one of said conductors and permitting longitudinal motion of the other conductor, wherein said fixed support comprises: a post-type insulator having a receptacle at each end, a conducting layer applied on the surface of said receptacles, at least one conducting means within said receptacles; and fastening means for maintaining the conducting means within each receptacle in electrical and mechanical contact with one of said conductors, and wherein said fastening means comprises: a stud member extending away from said insulator means and adapted to fit through a hole in said outer conductor a reinforcing plate member having a curvature substantially equal to that of the outer diameter of said outer conductor and having a hole for admitting said stud member, securing means cooperating with said stud member for providing a rigid and gastight connection of said stud member and reinforcing plate to said outer conductor.
 15. The gas-insulated transmission line as set forth in claim 11 wherein said first conductor fastening means comprises: a stud member extending away from said insulator means and adapted to fit through a hole in said outer conductor, a reinforcing plate member having a curvature substantially equal to that of the outer diameter of said outer conductor and having a hole for admitting said stud member; and securing means cooperating with said stud member for providing a rigid and gastight connection of said stud member and reinforcing plate to said outer conductor.
 16. In a gas-insulated transmission line comprising at least one modular unit, the combination of: a. a nonmagnetic outer conductor, b. a nonmagnetic inner conductor, c. means for insulatedly supporting said inner conductor within said outer conductor, and wherein said support means is responsive to longitudinal expansion of said inner conductor to maintain the longitudinal axis of said inner conductor substantially parallel to the longitudinal axis of said outer conductor, wherein said supporting means comprises: at least one fixed support for inhibiting longitudinal motion of aid outer conductor relative to said inner conductor, b. at least one movable support rigidly mounted to one of said coductors and permitting longitudinal motion of the other conductor, wherein said fixed support comprises: a post-type insulator having a receptacle at each end, a conducting layer applied on the surface of said receptacles, at least one conducting means within said receptacles; and fastening means for maintaining the conducting means within each receptacle in electrical and mechanical contact with one of said conductors, and wherein a predetermined portion of outer conductor has a larger diameter than the remaining portion of said outer conductor to create a low intensity field region and wherein the fastening means between said outer conductor and the associated conducting means is in said larger diameter portion.
 17. The gas-insulated transmission line as set forth in claim 11 wherein said movable support comprises: a post insulator having a receptacle at each end, a conducting layer applied to the surface of said receptacles, first condUcting means inserted within one of said conductors to provide an electrical and a rigid mechanical contact, second conducting means inserted within the other of said receptacles and having translational contact means cooperating with said other conductor to permit relative motion between conductors while said second conducting means is maintained in electrical contact with said other conductor. 